Love it :)
Luna is one of my favorite characters and I love Harry/Luna fics :) You have wonderful attention to detail in this story and make it very easy for the reader to picture the setting as well as whats going on.

Awwwm it started off so sweet, and then, it's so sad, that last line caps it. Excellent. Before that it's just kinda Luna, sweet, but ht elast line, so well administered, thansk for that, it was wonderful.
Ran

I think you might want to make this a little bit longer. Maybe not the letter longer, but take a little more time to describe this world around Luna, what's going on now? I also envisioned Luna as one of the more understanding people, and by asking some of those questions, put a little more emotion in it. She is a little floating in the clouds, and isn't your run-of-the-mill schoolgirl, and I question the opening statement of "Hi!" That seems a little peppy for her. I really like the last sentence of the letter though, that seems to really fit with Luna's character.